The invention concerns an actively shielded superconducting magnet coil system for generating a magnetic field in a working volume, which is designed to be rotationally symmetric about a z-axis, comprising a radially inner main field winding and a radially outer shielding winding, and an electrically highly conductive shielding cylinder (radially inner shielding cylinder) which is disposed radially inside the main field winding or between the main field winding and the shielding winding.
An actively shielded magnet coil system without radially inner shielding cylinder is disclosed e.g. in DE 100 33 411 A1. A magnet coil system which is not actively shielded comprising a radially inner shielding cylinder is disclosed e.g. in DE 39 00 725 A1.
Superconducting magnet coil systems are used to generate strong homogeneous magnetic fields in nuclear magnetic resonance (NMR) apparatuses.
Such magnet coil systems comprise a main field winding for generating the magnetic field in a working volume and a shielding winding to reduce stray fields in the surroundings of the magnet during operation thereby reducing the region of danger around the magnet coil system. The magnet coil systems moreover typically comprise an electrically conductive shielding cylinder radially inside the main field winding or between the main field winding and the shielding winding to protect the working volume from external magnetic field disturbances and keep the magnetic field in the working volume constant.
In case of a sudden transition of the main field coil into the normally conducting state (quench), this shielding cylinder (referred to as radially inner shielding cylinder below) produces a substantial sudden fringe field in the external region of the magnet coil system. This fringe field flash endangers technical systems and people, e.g. people having a pacemaker, in the region of the magnet coil system.
It is therefore the underlying purpose of the present invention to further develop a magnet coil system such that a fringe field flash in the external region of the magnet coil system is prevented in case of a quench.